NASA tracks bodies from largest to smallest. Yes, "city killer" asteroids are being found a little late, but NASA has been tracking the ones that actually matter (planet or continent-scale destruction) and is moving from them right on down. Some things are a little harder to see (objects coming in from the direction of the sun) but there are even plans to see those. NPR's Science Friday had a great story on just this.

The problem is comets and the occasional missile hurled by Jupiter with little warning. Some of the big stuff can't be found just because it's very far away right now or hasn't been gravity assisted by passing Jupiter.